1. Field of the Invention
The present invention relates to a discharge lamp lighting apparatus, and particularly to a discharge lamp lighting apparatus used in various information equipments, such as a facsimile, a copier, a scanner, and the like, and adapted to provide illumination for reading documents.
2. Description of the Related Art
Recently, a light source for providing illumination for reading documents in various information equipments, such as a facsimile, a copier, a scanner, and the like, or a light source for a backlight device for use in a liquid crystal display (LCD) device is required to provide a high brightness, a long product life, and a high reliability. Consequently, a rare gas discharge lamp utilizing dielectric barrier discharge is increasingly used. The rare gas discharge lamp utilizing dielectric barrier discharge is found to have various advantages: for example, it has a high luminous efficiency, and it has no electrodes in the discharge space thus preventing decrease of its product life due to sputtering discharge. An apparatus to light such a rare gas discharge lamp conventionally incorporates a push-pull inverter circuit (refer, for example, to Japanese Patent Application Laid-Open No. 2001-160497).
FIG. 4 is a block diagram of a typical example of such a discharge lamp lighting apparatus as mentioned above. Referring to FIG. 4, a discharge lamp lighting apparatus 104 includes a control circuit 106, a step-up circuit 105, a switch circuit 107, a step-up transformer 118, and a smoothing capacitor 115, wherein an input voltage Vi from an external DC voltage supply is stepped by the step-up circuit 105 up to a voltage Vj (boosted voltage) which, while supplied to the smoothing capacitor 115, is adapted to drive the primary side of the step-up transformer 118 by means of the switch circuit 107 thereby lighting a discharge lamp 101 connected at the secondary side of the step-up transformer 118. During this operation, the control circuit 106, according to a feedback signal FB of the boosted voltage Vj, outputs a control signal GC for the step-up circuit 105 and a control signal GS for the switch circuit 107, thus both the step-up operation of the step-up circuit 105 and the switching operation of the switch circuit 107 are controlled only by one single circuit, namely the control circuit 106.
FIG. 5 is a circuit diagram showing a specific circuitry of the discharge lamp lighting apparatus 104 of FIG. 4. The step-up circuit 105 is a step-up chopper circuit which includes a choke coil 112 connected in series to the positive terminal of the DC voltage supply, a diode 114 connected in series to the choke coil 112, and a switching element 113 constituted by, for example, a power MOSFET and connected between ground and the connection point of the choke coil 112 and the diode 114, and which has its output terminal connected to the smoothing capacitor 115 to smooth the boosted voltage Vj. The step-up circuit 105 further includes a gate resistor 133 connected to the gate terminal of the switching element 113, and, in order to rapidly charge and discharge the gate capacitance to thereby achieve a high-speed switching operation, a differentiation circuit 136 composed of a capacitor 131 and a resistor 132 is connected to the gate resistor 133, and then a buffer circuit 135, which is structured such that the emitter and base terminals of an NPN transistor 129 are connected respectively to the emitter and base terminals of a PNP transistor 130, is connected to the differentiation circuit 136.
The switch circuit 107 includes two switching elements 116 and 117 which are each constituted by, for example, a power MOSFET, and which are connected respectively to the both terminals of the primary winding of the step-up transformer 118. The midpoint tap of the primary winding of the step-up transformer 118 is connected to the boosted voltage Vj, and the switching elements 116 and 117 and the step-up transformer 118 make up a push-pull inverter circuit. The switch circuit 107 further includes gate resistors 124 and 125 connected to the respective gate terminals of the switching elements 116 and 117, and the differentiation circuit 136 and the buffer circuit 135 are provided for each of the switching elements 116 and 117 in the same way as in the switching element 113 of the step-up circuit 105.
The control circuit 106 includes a PWM (pulse width modulation) control circuit 119, a switch circuit gate signal generating circuit composed of transistors 120 and 121 disposed at the output stage of the PWM control circuit 119 and resistors 122 and 123, and a step-up circuit gate signal generating circuit including a signal adder composed of diodes 126 and 127 and a resistor 128.
The PWM control circuit 119 includes an error amplifier 137 which receives a feedback signal FB generated such that the boosted voltage Vj is divided by a voltage dividing circuit 111 so as to be fed back, and a reference signal generated such that a reference voltage Vref is divided. A control pulse generated in the PWM control circuit 119 is pulse width modulated according to the comparison of the feedback signal FB with the reference signal thereby turning into switch circuit gate signals GL and GU to be inputted to the respective gate terminals of the switching elements 116 and 117 of the switch circuit 107, and into a step-up circuit gate signal GC to be inputted to the gate terminal of the switching element 113 of the step-up circuit 105.
The operation of the discharge lamp lighting apparatus 104 described above will be explained with reference to FIGS. 6A to 6F showing sequence charts. Referring first to FIG. 6A, when the input voltage Vi from the external DC voltage supply is applied to the step-up circuit 105 at time t0, the control circuit 106 starts its operation, whereby gate signals B and D are outputted respectively to the switching element 113 of the step-up circuit 105 and to the switching elements 116 and 117 of the switch circuit 107 as shown in FIGS. 6D and 6F, and the switching elements 113, 116 and 117 are caused to start their on-off operations. When the switching elements 113 of the step-up circuit 105 transits from an on-state to an off-state, an induction voltage is generated at the choke coil 112, whereby the boosted voltage Vj is generated across the both terminals of the smoothing capacitor 115 as shown in FIG. 6B.
When the input voltage Vi is applied to the step-up circuit 105, an input current A starts to flow with a large rush current generated instantaneously as shown in FIG. 6C. If the DC voltage supply does not have a sufficient supply capacity to cover current excess resulting from superposition of the rush current on the input current A, then the input voltage Vi of the DC voltage supply is lowered below the rated voltage. Generally, for preventing the lowering of the voltage of a DC power supply, the DC power supply is required to have an increased supply capacity, which results in an increased dimension of the DC power supply.
Further, since the gate signal B for the switching element 113 of the step-up circuit 105, and the gate signal D and the gate signal respectively for the switching elements 116 and 117 of the switch circuit 107 start their operations simultaneously, the rush current flowing into the smoothing capacitor 115 becomes large. Consequently, an output current C with a rush current superposed thereon flows in the discharge lamp 101 as shown in FIG. 6E, and therefore a stress on electrodes 103 and 103′ of the discharge lamp 101 increases thus shortening the life of the discharge lamp 101. Also, generally, for reducing the rush current flowing into the discharge lamp 101 to any extent at all, delay operation must be performed in the step-up circuit 5, and consequently the current in the discharge lamp 101 inevitably has a current waveform with a delayed rise.
A conventional discharge lamp lighting apparatus generally includes an on-off switching circuit and a constant-voltage circuit both provided between a DC voltage supply and a control circuit, and is structured such that an input voltage from the DC voltage supply is preliminarily applied for putting a step-up circuit in a standby-state so that when the on-off switching circuit is turned on, the step-up circuit and the switch circuit are caused to start their operations so as to start lighting a discharge lamp. In such a discharge lamp lighting apparatus, since an input voltage Vi is preliminarily applied to a smoothing capacitor in a standby-state, the voltage at the smoothing capacitor rises from the input voltage Vi, not from 0 V, up to a prescribed boosted voltage Vj for lighting the discharge lamp. Consequently, the rush current is reduced when compared with the discharge lamp lighting apparatus 104 in which the voltage at the smoothing capacitor 115 rises from 0 V up to the boosted voltage Vj for lighting the discharge lamp 101.
However, it is still the case with the above-described conventional discharge lamp lighting apparatus that the step-up circuit and the switch circuit starts their operations simultaneously for lighting the discharge lamp, and the output current composed such that the rush current flowing into the smoothing capacitor is superposed on the input current is caused to flow in the discharge lamp thus raising the same problem as the discharge lamp lighting apparatus 104.